This invention relates to improved functional fluid compositions containing epoxides useful as acid scavengers. This invention further relates to phosphate ester-based functional fluids, particularly phosphate ester-based hydraulic fluids, containing the epoxide acid scavengers of this invention.
In the past, functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter medium for air-conditioning systems. Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures.
U.S. Pat. Nos. 3,723,320, 3,941,708, and 5,464,551 disclose epoxide acid scavengers for use in phosphate ester-based functional fluids and teach that the preferred epoxides are 3,4-epoxycycloalkyl carboxylates. These 3,4-epoxycycloalkyl carboxylates are typically used in current commercial phosphate ester-based functional fluid compositions and these fluids are susceptible to formation of carboxylic acids during use. During use of phosphate ester-based functional fluids, carboxylic acid levels increase and cause the useful life of the fluid to be reduced. While the '320 patent discloses —CH2OR and —C(O)R substituted epoxycyclohexyl compounds as one of numerous epoxycyclohexyl compounds in its generic epoxide formula, and the '708 patent discloses —CH2OR substituted epoxycycloalkyl compounds as one of numerous epoxycycloalkyl compounds in its generic epoxide formula, neither patent discloses or suggests either the problem of carboxylic acid formation nor a solution for this problem.
There exists a need for improved epoxide acid scavengers and for improved phosphate ester-based functional fluids containing such epoxide acid scavengers that generate less carboxylic acid during use of the functional fluids. There also exists a need for such improved epoxide acid scavengers to have an acceptable rate of depletion when compared to the currently used 3,4-epoxycycloalkyl carboxylates. It has now been discovered that the phosphate ester-based functional fluid compositions of the invention containing the epoxides of the invention generate less carboxylic acids during use of the functional fluids while having an acceptable depletion rate during use.